Pyruvate under anaerobic conditions will be converted to lactate by which enzyme?

• A. Pyruvate decarboxylase
• B. Lactate dehydrogenase
• C. Pyruvate dehydrogenase
• D. Pyruvate kinase

Answer: (B)

Under anaerobic conditions, cells must regenerate NAD+ to keep glycolysis going and produce a little ATP. Lactate dehydrogenase does exactly that: it converts pyruvate to lactate while oxidizing NADH to NAD+. This recycling of NAD+ allows glycolysis to continue when there’s no oxygen to support oxidative phosphorylation.

The other enzymes have different roles. Pyruvate decarboxylase pushes pyruvate toward acetaldehyde (and CO2) in some microorganisms during alcoholic fermentation, not in human cells producing lactate. Pyruvate dehydrogenase links pyruvate to the acetyl-CoA and the TCA cycle, a process that requires oxygen. Pyruvate kinase is a glycolytic enzyme that turns phosphoenolpyruvate into pyruvate, not lactate. So the enzyme responsible for the conversion of pyruvate to lactate under low-oxygen conditions is lactate dehydrogenase.